Performance Optimizer

What this skill does

# Performance Optimizer

Make applications fast, scalable, and cost-efficient.

## Core Principle

**Measure first, optimize second.** Don't guess at bottlenecks—profile, measure, then fix the slowest parts.

## Performance Budget

### Web Vitals (Target Metrics)

```yaml
Core Web Vitals:
  Largest Contentful Paint (LCP): < 2.5s # Main content visible
  First Input Delay (FID): < 100ms # Interaction responsiveness
  Cumulative Layout Shift (CLS): < 0.1 # Visual stability

Additional Metrics:
  First Contentful Paint (FCP): < 1.8s # First content rendered
  Time to Interactive (TTI): < 3.8s # Fully interactive
  Total Blocking Time (TBT): < 200ms # Main thread blocked
  Speed Index: < 3.4s # Visual progress

Backend Metrics:
  API Response Time (P95): < 500ms
  Database Query Time (P95): < 100ms
  Server Response Time (TTFB): < 600ms
```

---

## Phase 1: Profiling & Measurement

**Goal**: Identify actual bottlenecks, not perceived ones

### Frontend Profiling

**Chrome DevTools**:

```javascript
// 1. Performance tab → Record → Reload page
// 2. Analyze:
//    - Main thread activity
//    - Network waterfall
//    - JavaScript execution time
//    - Rendering time

// 3. Lighthouse audit
// Run: chrome://lighthouse or `npm i -g lighthouse`
lighthouse https://yoursite.com --view
```

**React DevTools Profiler**:

```javascript
// Wrap component to profile
import { Profiler } from 'react'

function onRenderCallback(id, phase, actualDuration) {
  console.log(`${id} (${phase}) took ${actualDuration}ms`)
}

;<Profiler id="ExpensiveComponent" onRender={onRenderCallback}>
  <ExpensiveComponent />
</Profiler>
```

### Backend Profiling

**Node.js Profiling**:

```bash
# Generate CPU profile
node --prof app.js

# Process profile
node --prof-process isolate-0x*.log > processed.txt

# Flame graphs (better visualization)
npm i -g 0x
0x app.js
```

**Python Profiling**:

```python
import cProfile
import pstats

# Profile function
cProfile.run('slow_function()', 'output.prof')

# Analyze
p = pstats.Stats('output.prof')
p.sort_stats('cumulative').print_stats(20)
```

### Database Profiling

**PostgreSQL**:

```sql
-- Enable query logging
ALTER DATABASE yourdb SET log_min_duration_statement = 100; -- Log queries >100ms

-- Analyze query
EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM users WHERE email = '[email protected]';

-- Find slow queries
SELECT query, mean_exec_time, calls
FROM pg_stat_statements
ORDER BY mean_exec_time DESC
LIMIT 20;
```

**MongoDB**:

```javascript
// Enable profiling
db.setProfilingLevel(1, { slowms: 100 })

// View slow queries
db.system.profile.find({ millis: { $gt: 100 } }).sort({ ts: -1 })

// Explain query
db.collection.find({ email: '[email protected]' }).explain('executionStats')
```

---

## Phase 2: Database Optimization

### Add Strategic Indexes

```sql
-- Before: Table scan (slow)
SELECT * FROM users WHERE email = '[email protected]';
-- Execution time: 2000ms on 1M rows

-- After: Index scan (fast)
CREATE INDEX idx_users_email ON users(email);
SELECT * FROM users WHERE email = '[email protected]';
-- Execution time: 5ms

-- Composite index for multi-column queries
CREATE INDEX idx_posts_user_date ON posts(user_id, created_at DESC);
SELECT * FROM posts WHERE user_id = 123 ORDER BY created_at DESC;

-- Partial index for filtered queries
CREATE INDEX idx_active_users ON users(created_at) WHERE is_active = true;
```

### Eliminate N+1 Queries

```typescript
// ❌ Bad: N+1 query problem (101 database queries)
const users = await User.findAll() // 1 query
for (const user of users) {
  user.posts = await Post.findAll({ where: { userId: user.id } }) // N queries
}

// ✅ Good: Eager loading (2 queries)
const users = await User.findAll({
  include: [{ model: Post }]
})

// ✅ Better: DataLoader (batching + caching)
const userLoader = new DataLoader(async userIds => {
  const users = await User.findAll({ where: { id: userIds } })
  return userIds.map(id => users.find(u => u.id === id))
})
```

### Query Optimization

```sql
-- Avoid SELECT *
-- ❌ Bad
SELECT * FROM users WHERE id = 1;

-- ✅ Good
SELECT id, name, email FROM users WHERE id = 1;

-- Use LIMIT
-- ❌ Bad
SELECT * FROM posts ORDER BY created_at DESC;

-- ✅ Good
SELECT * FROM posts ORDER BY created_at DESC LIMIT 20;

-- Avoid functions in WHERE clause
-- ❌ Bad (can't use index)
SELECT * FROM users WHERE LOWER(email) = '[email protected]';

-- ✅ Good (can use index)
SELECT * FROM users WHERE email = '[email protected]';
-- Store email as lowercase, or use generated column + index
```

### Connection Pooling

```typescript
// PostgreSQL connection pool
import { Pool } from 'pg'

const pool = new Pool({
  max: 20, // Maximum connections
  min: 5, // Minimum connections
  idleTimeoutMillis: 30000, // Close idle connections after 30s
  connectionTimeoutMillis: 2000 // Error if can't connect in 2s
})

// Always release connections
const client = await pool.connect()
try {
  const result = await client.query('SELECT * FROM users')
  return result.rows
} finally {
  client.release()
}
```

---

## Phase 3: Caching Strategy

### Multi-Layer Caching

```
Browser Cache (HTTP headers)
  ↓
CDN Cache (Cloudflare, CloudFront)
  ↓
Application Cache (Redis, Memcached)
  ↓
Database Query Cache
  ↓
Database
```

### Redis Caching

```typescript
import Redis from 'ioredis'

const redis = new Redis({
  maxRetriesPerRequest: 3,
  enableReadyCheck: true
})

async function getUser(id: string): Promise<User> {
  const cacheKey = `user:${id}`

  // 1. Check cache
  const cached = await redis.get(cacheKey)
  if (cached) {
    return JSON.parse(cached)
  }

  // 2. Cache miss - fetch from database
  const user = await db.users.findById(id)

  // 3. Store in cache (expire in 1 hour)
  await redis.setex(cacheKey, 3600, JSON.stringify(user))

  return user
}

// Cache invalidation
async function updateUser(id: string, data: Partial<User>) {
  await db.users.update(id, data)
  await redis.del(`user:${id}`) // Invalidate cache
}
```

### HTTP Caching Headers

```typescript
// Express middleware
app.use((req, res, next) => {
  // Static assets: cache for 1 year
  if (req.url.match(/\.(js|css|png|jpg|jpeg|gif|svg|woff|woff2)$/)) {
    res.setHeader('Cache-Control', 'public, max-age=31536000, immutable')
  }

  // HTML: no cache (always revalidate)
  if (req.url.endsWith('.html') || req.url === '/') {
    res.setHeader('Cache-Control', 'no-cache, must-revalidate')
  }

  // API responses: cache for 5 minutes
  if (req.url.startsWith('/api/')) {
    res.setHeader('Cache-Control', 'public, max-age=300')
    res.setHeader('ETag', generateETag(req.url))
  }

  next()
})
```

### CDN Configuration

```yaml
Static Assets to CDN:
  - Images: /images/**
  - JavaScript: /js/**
  - CSS: /css/**
  - Fonts: /fonts/**

CDN Settings:
  - Cache duration: 1 year (with versioned URLs)
  - Gzip/Brotli compression: enabled
  - Image optimization: WebP conversion
  - Purge on deploy: yes (via API)

Recommended CDNs:
  - Cloudflare (free tier excellent)
  - CloudFront (AWS integration)
  - Fastly (enterprise, very fast)
```

---

## Phase 4: Frontend Optimization

### Code Splitting & Lazy Loading

```typescript
// React lazy loading
import { lazy, Suspense } from 'react'

// ❌ Bad: Load everything upfront
import Dashboard from './Dashboard'
import AdminPanel from './AdminPanel'

// ✅ Good: Lazy load routes
const Dashboard = lazy(() => import('./Dashboard'))
const AdminPanel = lazy(() => import('./AdminPanel'))

function App() {
  return (
    <Suspense fallback={<LoadingSpinner />}>
      <Routes>
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/admin" element={<AdminPanel />} />
      </Routes>
    </Suspense>
  )
}

// Next.js dynamic imports
import dynamic from 'next/dynamic'

const HeavyComponent = dynamic(() => import('./HeavyComponent'), {
  loading: () => <LoadingSpinner />,
  ssr: false // Skip SSR for this component
})
```

### Image Optimization

```jsx
// Next.js Image component (automatic optimization